A standard flow-control valve can be controlled by a motor unit that is connected to the shaft of the valve and that incorporates a potentiometer whose resistance is an analog output corresponding to valve position. This analog output can be electronically evaluated to determine if the valve is open or closed, and just what intermediate position it may be in.
In German patent documents 3,518,645 and 4,104,318 these systems operate by a method whereby on initial installation the position-reading mechanism is zeroed, that is set to read the closed position of the valve, by the installer. This is a fairly complex calibrating system that must be carried out by an experienced worker. Since the angular movement of a valve from full closed to fully open, or from full cold to full hot depending on the type of valve, is constant for a given type of valve, the exact location of the fully closed position is extremely critical.
In addition a problem with known valves is that, with time, the valve wears somewhat and the fully closed position can actually move slightly, so that when the valve controller detects that the valve is closed, for instance, it is still open a little. Similarly in some of the systems the valve can get stiffer so that it cannot move into the fully closed position and the servomotor can burn out trying to do so. Both problems require complex recalibration of the valve, something that only a trained person can do.